# Leaf temperature

> Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.

- Canonical: https://genetixbay.com/knowledge/glossary/leaf-temperature.html
- Also written: leaf temp, leaf surface temperature, canopy temperature, leaf temperature offset
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Leaf temperature is the temperature of the leaf tissue itself. It is set by a balance of energy gained (absorbed light and radiant heat) and lost ([transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md) cooling, convection to air, radiation to cooler surfaces). It is the temperature that governs [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) rate and leaf [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), so it matters more than the room thermometer.

## In practice

Under [LED](https://genetixbay.com/knowledge/glossary/led.md), which emits little radiant heat, a well-watered, transpiring leaf often sits 1–3 °C (2–5 °F) below air temperature. Under [HPS](https://genetixbay.com/knowledge/glossary/hps.md), the extra infrared often pushes leaves to or above air temperature. Nelson and Bugbee (2015) quantified this. It is the main reason a grower who switches from HPS to LED should raise room temperature: the goal is a similar leaf temperature, typically 24–28 °C (75–82 °F) in flower.

Leaf temperature rising towards or above air temperature under LED can mean [stomata](https://genetixbay.com/knowledge/glossary/stomata.md) are closing, a sign of root problems, drought or VPD set too high.

## Don't confuse with

- **Air temperature**: what the room thermometer reads.
- **Canopy-top temperature near the fixture**: can be hotter than leaves lower down.
- **Low-stress training (LST)**: an unrelated training method sharing the abbreviation; prefer "leaf temp" to avoid confusion.

## Related terms

[Leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md), [IR thermometer](https://genetixbay.com/knowledge/glossary/ir-thermometer.md), [Temperature](https://genetixbay.com/knowledge/glossary/temperature.md), [LED vs HPS](https://genetixbay.com/knowledge/glossary/led-vs-hps.md), [Heat stress](https://genetixbay.com/knowledge/glossary/heat-stress.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)

## Sources

1. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/